This application claims the priority of German application No. 19643053.4-13, filed in Germany on Oct. 18, 1996 the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for reducing nitrogen oxide emissions from a direct-injected gasoline engine operated in broad characteristic field ranges with stratified charging by recirculating exhaust gas during low load conditions and operating the engine with homogenous carburetion and less circulation during high load conditions.
Direct injection of fuel into the cylinders considerably reduces fuel consumption by comparison with other operating methods for a gasoline engine. During the compression stroke, lean combustion takes place during stratified charge operation with fuel injection. Recirculation of a portion of the exhaust stream tapped off the exhaust line and feeding it into the fresh charge line of the engine is a proven method for reducing pollutant emissions, but the cleaning power of this measure is insufficient and the pollutant content of the exhaust that is finally expelled usually is significantly over the legal limit. Catalytic converters located in the exhaust line are conventionally employed to reduce the emissions content in the exhaust but, as in the case of the three-way catalytic converter that is considered to be the most efficient, satisfactory pollutant reduction is only possible at nearly stoichiometric air ratios in a narrow lambda window of approximately lambda=1. During stratified charge operation of a direct-injected gasoline engine with air ratios whose lambda values are significantly greater than 1 and can be as high as approximately 10, catalytic reduction of the nitrogen oxide molecules in the oxygen-rich exhaust is not possible. One known method for processing gases that are rich in NOx is a storage catalytic converter called the DeNOx catalytic converter, which adsorbs the nitrogen oxide molecules that cannot be reduced in oxygen-rich gases. Thus, during stratified charge operation of a direct-injected engine, the nitrogen oxide emissions can be stored in the catalytic converter. To reduce the stored nitrogen oxide molecules to nitrogen, shortly before the adsorption capacity limit of the storage catalytic converter is reached, at the latest, the operating mode of the engine must be adjusted so that oxygen-poor exhaust is formed during combustion, with stoichiometric air ratios (lambda=1).
Previously it was assumed that oxygen-poor exhaust with stoichiometric air ratios of lambda=1 could be obtained during combustion only if homogeneous carburetion was performed. If the engine is operated with the stratified charging that is advantageous in the partial load range, with fuel injection during the compression stroke, it is necessary to switch from this operating mode to operation with homogeneous carburetion, in other words, fuel injection during the intake stroke. During the change from stratified charging to homogeneous carburetion, the engine operates for several working cycles with a fuel/air mixture that is both homogeneous and lean, because of the shift in injection timing. This favors the formation of new nitrogen oxides and a temporary extremely high nitrogen oxide emission occurs that cannot be completely adsorbed by the storage catalytic converter. Moreover, the homogeneous lean mixture is insufficiently ignitable, so that misfiring must be anticipated during the switch from injection during the compression stroke to injection during the intake stroke. Brief rough operation of the engine cannot be prevented because of the asymmetry of the ignition timing during the change from one operating state to the other.
Hence a goal of the invention is to provide a method for reducing nitrogen oxide emissions from a direct-injected gasoline engine, said method being performed during continuous stratified charge operation of the engine.
This goal is achieved according to embodiments of the present invention.
During stratified charge operation of the engine, the exhaust gas recirculation rate into the fresh charge line is increased for an interval of time before the adsorption capacity limit of the storage catalytic converter is reached. Thus, depending on the operating point of the engine, a quantity of exhaust is recirculated such that a stoichiometric composition of the exhaust results following combustion. During this lambda window produced by stratified charge operation, the nitrogen oxide molecules collected in the storage catalytic converter are desorbed and catalytically converted. The duration of the operating interval of the engine with stoichiometric combustion as a result of increased exhaust recirculation depends on the quantity of nitrogen oxide molecules to be reduced, in other words, on the design of the storage catalytic converter in which the nitrogen oxide molecules are adsorbed during stratified charge operation. Desorption of the nitrogen oxide molecules in the storage catalytic converter thus takes place completely during stratified charge operation of the engine with a high exhaust gas recirculation rate, so that reduction in the storage catalytic converter is accelerated by low nitrogen oxide raw emission during combustion.
A throttle valve is located in the fresh charge line upstream of the point where the exhaust recirculation line joins it, the position of said valve regulating the fresh charge flow though the fresh charge line. The composition of the combustion air that is recycled to the cylinders of the engine and is composed of fresh air and recirculated exhaust can thus be regulated by both the exhaust recirculation valve in the exhaust recirculation line and the throttle valve. During stratified charge operation of the internal combustion engine with a low load, the desired exhaust recirculation rate for achieving stoichiometric conditions during combustion with the exhaust gas recirculation valve fully open is controlled by the position of the throttle valve. An adjustable throttle valve reduces the fresh air flow and by producing a vacuum in the fresh charge line, produces a pressure drop between the exhaust line and the fresh charge line that drives exhaust through the exhaust gas recirculation line. In this load range, during stratified charge operation of the internal combustion engine, high exhaust gas recirculation rates are required to achieve stoichiometric air ratios whose delivery is made possible by reducing the fresh air flow.
During stratified charge operation of the internal combustion engine with a high load, the required exhaust gas recirculation rate for producing stoichiometric conditions is controlled by the setting of the opening of the exhaust gas recirculation valve in the exhaust gas recirculation line. Lower exhaust gas recirculation rates are required than for stratified charge operation with a low load, so that the throttle valve is advantageously fully open.
The throttle valve and/or the exhaust gas recirculation valve are especially advantageously individually adjustable by an electronic control unit at each operating point of the engine. As a result of the fresh air flow and/or the recycled exhaust stream settings, stoichiometric conditions are created at every operating point of the engine. The control unit uses a characteristic map to determine the setting parameters stored therein for the respective operating point of the engine and, as a function of these parameters, generates a control signal that is fed to the throttle valve and/or the exhaust gas recirculation valve. Advantageously the control signal can be optimized by either an oxygen sensor in the fresh charge line or a nitrogen oxide sensor in the exhaust line generating a measurement signal that provides information on the oxygen content in the fresh charge or the nitrogen oxide content in the exhaust line, said signal being continuously transmitted to the control unit. When the measurement signal varies from the range of the desired stoichiometric window, the control unit corrects the control signal accordingly, adjusting the exhaust gas recirculation rate.
In an improvement on the invention, at a high operating load or during full load operation of the engine, following fuel injection into the cylinders, homogeneous carburetion takes place which in this load range of the engine results in a higher power output. One advantageous property of homogeneous combustion is the production of exhaust gases with a stoichiometric composition, so that nitrogen emissions can be broken down catalytically without preliminary measures.
Advantageously the exhaust line is divided into branches guided parallel to one another, with the storage catalytic converter being located in one branch and a multifunctional catalytic converter being located in the other branch. A controllable distributing valve is located in the entrance to the branches, with the exhaust from the engine flowing through one of the branches depending on the position of said valve. The distributing valve advantageously can be set so that the exhaust is conducted fully through one of the branches and hence through one of the catalytic converters, while the other branch is shutoff in each case. During stratified charge operation of the engine, the distributing valve is set so that the exhaust flows completely through the partial load branch in which the storage catalytic converter is located. The nitrogen oxide molecules contained in oxygen-rich exhaust are adsorbed in the storage catalytic converter and according to the invention are desorbed during the production of stoichiometric exhaust compositions by a corresponding increase in the exhaust gas recirculation rate and converted catalytically. During full load operation of the engine with homogeneous carburetion, the distributing valve shuts off the partial load branch and opens the full load branch in which the multifunctional catalytic converter is located. Preferably a three-way catalytic converter is used here, in which selective catalytic reaction of the nitrogen oxides under the conditions that prevail during full load operation of the engine, namely exhaust compositions in a lambda window around the stoichiometric point and relatively high exhaust temperatures, proceed nearly simultaneously with other selective catalytic reactions involving other components of the exhaust.
Downstream from the catalytic converters, the two branches join to form a common section of the exhaust line, with a low structural cost being achieved for the exhaust line from the outlet of the engine unit the exhaust gases are finally expelled from the exhaust line. Advantageously, a shutoff valve is located in the partial load branch of the exhaust line, between the storage catalytic converter and the connection to the full load branch. If the partial load branch is opened by the distributing valve during stratified charge operation of the internal combustion engine, the shutoff valve opens as well. During full load operation, on the other hand, with the flow passing through the full load branch, the shutoff valve closes off the partial load branch downstream from the storage catalytic converter so that the hot exhaust flowing through the full load branch cannot enter the partial load branch and cause excessive harmful heating of the inactive storage catalytic converter.